Cancer is the 2nd leading cause of death in the United States1 and is projected to cost over $157 billion by 2020.2 Current cancer treatments involve procedures that can severely affect patient quality of life. High-intensity focused ultrasound (HIFU) is a non-invasive targeted tumor therapy modality that may address many of the shortcomings of radiation and chemotherapy.3 Monitoring and assessment of HIFU arguably remain the most significant obstacles to more widespread use and acceptance of HIFU therapy for cancer treatment. The objective of this application is to establish the feasibility of using quantitative ultrasound (QUS) to assess HIFU treatment of tumor tissues by investigating the hypothesis that ultrasound is sensitive to morphological changes in tumor tissue with HIFU treatment. The use of QUS to assess HIFU treatment is supported by studies in the literature using QUS to characterize cancerous and health tissues4-7 and to assess other cancer therapies, such as radiation and hyperthermia.8,9 The specific aims for this proposal are 1) To detect the acute effects of high-intensity focused ultrasound therapy in rat tumors in vivo using quantitative ultrasound and 2) To monitor rat tumor cell death in vivo after high-intensity focused ultrasound therapy using quantitative ultrasound. The proposed research will provide validation of QUS assessment as a viable modality for HIFU therapy feedback. This research will benefit medical scientists and clinicians by providing a better understanding of ultrasound sensitivity to tumor tissue morphology after HIFU therapy. Thus, it represents an important step towards clinical acceptance of HIFU cancer therapy, and therefore a better prognosis and quality of life for cancer patients.